Personal health monitors provide users with the ability to monitor their overall health and fitness by enabling the user to monitor heart rate or other physiological information during exercise, athletic training, rest, daily life activities, physical therapy, etc. Such devices are becoming increasingly popular as they become smaller and more portable.
A heart rate monitor represents one example of a personal health monitor. A common type of heart rate monitor uses a chest strap that includes surface electrodes to detect muscle action potentials from the heart. Because such surface electrodes provide a relatively noise free signal, the information produced by monitors that use surface electrodes is relatively accurate. However, most users find chest strap monitors uncomfortable and inconvenient.
Another type of monitor uses photoplethysmograph (PPG) sensors disposed in an ear bud. The ear provides an ideal location for a monitor because it is a relatively immobile platform that does not obstruct a person's movement or vision. PPG sensors proximate the ear may have, e.g., access to the inner ear canal and tympanic membrane (for measuring core body temperature), muscle tissue (for monitoring muscle tension), the pinna and earlobe (for monitoring blood gas levels), the region behind the ear (for measuring skin temperature and galvanic skin response), and the internal carotid artery (for measuring cardiopulmonary functioning). The ear is also at or near the point of the body's exposure to environmental breathable toxins of interest (volatile organic compounds, pollution, etc.), noise pollution experienced by the ear, lighting conditions for the eye, etc. Further, as the ear canal is naturally designed for transmitting acoustical energy, the ear provides a good location for monitoring internal sounds, such as the heartbeat, breathing rate, mouth motion, etc.
PPG sensors measure the relative blood flow using an infrared or other light source that projects light that is ultimately transmitted through or reflected off tissue, and is subsequently detected by a photodetector and quantified. For example, higher blood flow rates result in less light being absorbed, which ultimately increases the intensity of the light that reaches the photodetector. By processing the signal output by the photodetector, a monitor using PPG sensors may measure the blood volume pulse (the phasic change in blood volume with each heartbeat), the heart rate, heart rate variability, and other physiological information.
PPG sensors are generally small and may be packaged such that they do not encounter the comfort and/or convenience issues associated with other conventional health monitors. However, PPG sensors are also highly sensitive to noise, and thus are more prone to accuracy problems. For example, a motion component of a user, e.g., a step rate of a jogger, is often as strong as or stronger than a heart rate component, which may corrupt a heart rate measurement. U.S. Pat. No. 7,144,375, which discloses using an accelerometer as a motion reference for identifying the potential step rate component(s) of a PPG sensor output, provides one possible solution to this problem. When the step rate is close to the heart rate, the '375 patent teaches spectrally transforming the step rate and heart rate waveforms, e.g., over a window of samples, respectively provided by the step rate and heart rate sensors to create a step rate spectrum and a heart rate spectrum. If the spectral transform operation uses a 6 s window, the average latency incurred for the transform operation is 3 s. After performing the spectral transform, the '375 patent spectrally subtracts the heart rate and step rate spectrums. The '375 patent further keeps a history of the top ten peaks from the output of the spectral subtraction to perform various statistical analyses in order to achieve the desired accuracy before making a decision regarding whether there is a cross-over between the heart rate and the step rate, and before making a decision regarding which spectral peak corresponds to the heart rate. Thus, the post-transform operations implemented by the '375 patent incur an additional processing latency, e.g., of ten seconds, which is undesirable. Thus, there remains a need for alternative solutions that provide an accurate heart rate with less latency when the step rate is close to the heart rate.